In a case where an LED is used for a backlight of, for example, a liquid crystal television, the following configuration has been known (see Patent Literature 1). According to the configuration, a voltage source for turning on the LED is supplied by a booster circuit to the LED, and a pulse width modulation duty ratio (PWM-Duty) in which an electric current flowing to the LED is turned on/off is adjusted so that a desired light emitting brightness of the LED can be obtained.
FIG. 9 is a circuit diagram illustrating a configuration of a conventional LED drive device 1000. The LED drive device 1000 includes LEDs 300R through 300B and a power supply device 100 for driving the LEDs 300R through 300B. The LED drive device 1000 is driven by a battery 200, and the power supply device 100 generates a drive voltage Vout for driving the LEDs 300R through 300B by boosting a battery voltage supplied from the battery 200. When colors do not particularly need to be distinguished from each other, the following description omits indices R, G, and B which are attached to members so as to correspond to the respective colors.
The power supply device 100 has an input terminal 102 via which a battery voltage is inputted, an output terminal 104 which is connected with an anode terminal of an LED 300 and via which an output voltage Vout obtained by boosting the battery voltage is outputted, and an LED terminal 106 which is connected with a cathode terminal of the LED 300.
The power supply device 100 includes a booster circuit 60 and a drive control section 70. The booster circuit 60 boosts the battery voltage inputted via the input terminal 102 and outputs the output voltage Vout via the output terminal 104.
The booster circuit 60 serves as a switching power supply including switching elements such as a switching regulator and a charge pump circuit. The booster circuit 60 has an enable terminal EN. When the enable terminal EN receives an enable signal SIG12 which is at a high level, the booster circuit 60 boosts the battery voltage by carrying out a switching operation. Meanwhile, when the enable terminal EN receives the enable signal SIG12 which is at a low level, the booster circuit 60 stops the switching operation.
The drive control section 70 controls a drive state of each of the LEDs 300R through 300B. The drive control section 70 includes constant current circuits 22R through 22B, switches 24R through 24B, AND gates 26R through 26B, a brightness controlling PWM oscillator 30, a light emitting pattern generator 82, and an OR gate 34.
The light emitting pattern generator 82 controls turning on/off of the LEDs 300R through 300B in accordance with data stored in a memory or externally supplied data. The light emitting pattern generator 82 generates light emission control signals SIG10R through SIG10B corresponding to the respective colors. When each emission control signal SIG10 is at a high level, a corresponding LED 300 emits light. Meanwhile, when the each light emission control signal SIG10 is at a low level, the corresponding LED 300 stops emitting light.
A constant-current circuit 22 is connected with a cathode of the LED 300 via the LED terminal 106 and is provided on a path through which an electric current of each of the LEDs 300R through 300B flows. A switch 24 turns on/off electric current generation by the constant current circuit 22.
The brightness controlling PWM oscillator 30 generates a PWM signal SIG14 for turning on/off the switch 24. The brightness controlling PWM oscillator 30 includes a voltage comparator 40, an oscillator 42, and a reference voltage source 44. The reference voltage source 44 generates a reference voltage Vref corresponding to each of the colors R, G, and B. The oscillator 42 generates a periodic voltage Vosc having a triangular waveform or a saw-tooth waveform.
An AND gate 26 receives a light emission control signal SIG10 supplied from the light emitting pattern generator 82 and receives the PWM signal SIG14 supplied from the brightness controlling PWM oscillator 30. The AND gate 26 outputs a logical product of these two received signals in a form of an output signal SIG16. The output signal SIG16 of the AND gate 26 is at a high level when both the light emission control signal SIG10 and the PWM signal SIG14 are at a high level.
The OR gate 34 receives the three light emission control signals SIG10R through SIG10B supplied from the light emitting pattern generator 82. The OR gate 34 supplies a logical sum of these three received signals to the enable terminal EN of the booster circuit 60.
According to the LED drive device 1000 having the configuration, the booster circuit 60 supplies the drive voltage Vout to each of the plurality of LEDs 300R through 300B in the power supply device 100. The drive control section 70 controls the drive state, i.e., a light emitting brightness of each of the LEDs 300R through 300B. The drive control section 70 carries out time sharing driving with respect to a plurality of load circuits. Further, the booster circuit 60, which has the enable terminal EN, stops the switching operation during a non-light emitting period in which none of the LEDs 300R through 300B are driven by the drive control section 70.
The light emitting pattern generator 82 generates the light emission control signals SIG10R through SIG10B for instructing the respective LEDs 300 to emit light. The booster circuit 60 logically operates the light emission control signals SIG10R through SIG10B so as to stop the switching operation during the non-light emitting period in which none of the LEDs 300 emit light.
As described earlier, the LED drive device 1000 turns on/off the LEDs 300R through 300B by turning on/off the respective switches 24R through 24B, and is capable of reducing electric power consumption by stopping oscillation of the booster circuit 60 during turning off of the LEDs 300R through 300B.
FIG. 10 is a circuit diagram illustrating a configuration of another conventional LED drive device. The LED drive device includes a direct-current power supply 201, a voltage detecting device 202, a control circuit 203, an input switch circuit 204, an output switch circuit 205, a signal formation circuit 206, a booster circuit 207, and an electric current detection circuit 208.
The direct-current power supply 201 generates a substantially constant direct-current voltage as an output voltage and supplies an electric current in accordance with a load connected with the direct-current power supply 201. When the direct-current power supply 201 has an output voltage Vin which is lower than a given threshold, the voltage detecting device 202 outputs a control signal for intercepting electric power supply from the direct-current power supply 201 to the booster circuit 207. The control circuit 203 outputs a control signal for controlling each of the input switch circuit 204 and the output switch circuit 205. The signal formation circuit 206 reverses, from a high level to a low level or from the low level to the high level, a level of a turn on control signal applied to a terminal 110.
The booster circuit 207 boosts the power supply voltage Vin and causes the boosted power supply voltage Vin to be an output voltage, and conducts, to an LED group 211, an output electric current Io in accordance with the turn on control signal. For example, the electric current detection circuit 208 detects a voltage between both ends of each of resistances which are serially connected with the LED group 211, and supplies the voltage to a control circuit 271 of the booster circuit 207. The control circuit 271 carries out constant current control so that an average of electric currents flowing to the LED group 211 is constant.
The output switch circuit 205 is parallel-connected with the LED group 211, and in sync with the turn on control signal, forces attenuation of the output electric current flowing from the booster circuit 207 during a turn off period of the LED group 211.